Zen Cart has a great email archiving feature where, if it is enabled, every email sent through your cart is stored in a database table. 

However, Zen Cart doesn't include an admin utility to look up emails in this table. With this Zen Cart Addon you can view emails sent through your admin area.

Keep in mind that if this is turned on and your cart sends out a lot of emails, your database will get pretty large and you will need to purge emails periodically.

Works with versions 1.5.0, 1.5.1, 1.5.3, 1.5.4, 1.5.5